Welcome to the forum.
When I try to play video clips (all formats) in the Source and Program screens nothing happens.
Please give us complete details about the exact formats/CODEC's that you have tried. Many will work fine, and many will not. It depends on exactly what is in those files.
In general, make sure that you have updated your video drivers.
Tell us about your videocard, and the driver.
If you have Hardware Acceleration ON, try turning it OFF. If this works, step up, one tick mark at a time, until it does not - back off one tick mark.
Check your OpenGL settings.
If you have an nVidia card with a newer driver, check to see if you have the PhysX, or the 3D Stereoscopic modules. In the nVidia console, disable these. Start with the 3D Stereoscopic module and test.
Check your PrPro Preferences for Playback. Makesure you have those set correctly.
I have more than sufficient space on my hard drive
The singular case is very troublesome. At least 2 physical disks are required.
Look here and specifically at the Storage guide: How to get the best from a PC? Some guides...
First, work through all of the steps here http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with the DETAILS asked for in the questions at the end of that link